1. Field of the Invention
The present invention relates to an electric storage device, and its production method, and more particularly to a technique well adaptable to reduce and suppress the expansion of an electric storage device, which is caused by the generation of decomposed gas from a component of electrolyte solution, in case where a material having a weight-average mesopore/macropore specific surface area within a specific range is used for an electrode material.
2. Description of the Related Art
The technique described below has been studied for completing the present invention. The summary is as follows.
Nowadays, an environmental issue, specifically vehicle-exhaust gas emission, is widely talked about. In this situation, efforts are made of developing environment-friendly electric vehicles and the like. In the electric vehicle development, a strong development effort is focused on an electric storage device to be used as a power source. Many types of electric storage devices have been proposed for replacement of the conventional lead battery.
There is a lithium ion secondary battery or an electric double layer capacitor as the electric storage device. In particular, a hybrid capacitor including a lithium ion capacitor having lithium ions doped into a negative electrode beforehand is now attracting attention. Some of the hybrid capacitors are actually mounted to a vehicle, and an execution test is performed in order to put the hybrid capacitor into practical use. Meanwhile, a further development in the components of the electric storage device has been carried out.
The development of the components described above includes the development in an electrode material, for example. For example, Japanese Patent Application Laid-Open (JP-A) No. 2008-150270 describes that a carbon material obtained by burning a carbon precursor, which is an aggregate powder of a carbon particle to which a carbon black having an average particle diameter of 12 to 300 nm is bonded, is cheap and has an excellent charging/discharging characteristic as a negative electrode material. JP-A No. 2007-91557 discloses an invention relating to an improvement of a high input/output characteristic of a lithium ion secondary battery. JP-A No. 2007-91557 describes that a carbon material, in which a total specific surface area obtained according to a BET method and a specific surface area of mesopore area obtained according to a BJH method fall within the range of 10 to 40 m2/g respectively, and the ratio of the mesopore specific surface area to the total specific surface area is 0.7 or more, has an effect of improving the high input/output characteristic.
The present inventors have been studying an electrode material of an electric storage device for a long time. The present inventors have found in their study that the introduction of a concept of a weight-average mesopore/macropore specific surface area to a negative electrode material is effective for improving the input/output characteristic. When the weight-average mesopore/macropore specific surface area of a negative electrode material is set to fall within a specific range, an excellent charging/discharging characteristic, particularly an excellent input/output characteristic, can be obtained even with the materials described in JP-A Nos. 2008-150270 and 2007-91557. In other words, although the feature of the negative electrode material is set within the scope of the claims in JP-A Nos. 2008-150270 and 2007-91557, an expected effect cannot always be obtained. On the other hand, when the weight-average mesopore/macropore specific surface area of the negative electrode material is a specific value or more, the electric storage device might expand due to the generation of the decomposed gas from a component of electrolyte solution, in some cases. The significant expansion of the electric storage device, which is caused in some cases, might cause a leakage of the electrolyte solution that has corrosivity or a leakage of gas because the electric storage device is opened.